This invention relates to single-pass inkjet printing.
In typical inkjet printing, a print head delivers ink in drops from orifices to pixel positions in a grid of rows and columns of closely spaced pixel positions.
Often the orifices are arranged in rows and columns. Because the rows and columns in the head do not typically span the full number of rows or the full number of columns in the pixel position grid, the head must be scanned across the substrate (e.g., paper) on which the image is to be printed.
To print a full page, the print head is scanned across the paper in a head scanning direction, the paper is moved lengthwise to reposition it, and the head is scanned again at a new position. The line of pixel positions along which an orifice prints during a scan is called a print line.
In a simple scheme suitable for low resolution printing, during a single scan of the print head adjacent orifices of the head print along a stripe of print lines that represent adjacent rows of the pixel grid. After the stripe of lines is printed, the paper is advanced beyond the stripe and the next stripe of lines is printed in the next scan.
High-resolution printing provides hundreds of rows and columns per inch in the pixel grid. Print heads typically cannot be fabricated with a single line of orifices spaced tightly enough to match the needed printing resolution.
To achieve high resolution scanned printing, orifices in different rows of the print head can be offset or inclined, print head scans can be overlapped, and orifices can be selectively activated during successive print head scans.
In the systems described so far, the head moves relative to the paper in two dimensions (scanning motion along the width of the paper and paper motion along its length between scans).
Inkjet heads can be made as wide as an area to be printed to allow so-called single-pass scanning. In single-pass scanning, the head is held in a fixed position while the paper is moved along its length in an intended printing direction. All print lines along the length of the paper can be printed in one pass.
Single-pass heads may be assembled from linear arrays of orifices. Each of the linear arrays is shorter than the full width of the area to be printed and the arrays are offset to span the full printing width. When the orifice density in each array is smaller than the needed print resolution, successive arrays may be staggered by small amounts in the direction of their lengths to increase the effective orifice density along the width of the paper. By making the print head wide enough to span the entire breadth of the substrate, the need for multiple back and forth passes can be eliminated. The substrate may simply be moved along its length past the print head in a single pass. Single-pass printing is faster and mechanically simpler than multiple-pass printing.
Theoretically, a single integral print head could have a single row of orifices as long as the substrate is wide. Practically, however, that is not possible for at least two reasons.
One reason is that for higher resolution printing (e.g., 600 dpi), the spacing of the orifices would be so small as to be mechanically unfeasible to fabricate in a single row, at least with current technology. The second reason is that the manufacturing yield of orifice plates goes down rapidly with increases in the number of orifices in the plate. This occurs because there is a not insignificant chance that any given orifice will be defective in manufacture or will become defective in use. For a print head that must span a substrate width of, say, 10 inches, at a resolution of 600 dots per inch, the yield would be intolerably low if all of the orifices had to be in a single orifice plate.
In general, in one aspect, the invention features a single-pass ink jet printing head having an array of ink jet outlets sufficient to cover a target width of a print substrate at a predetermined resolution. There are multiple orifice plates each having orifices. Each of the orifice plates serves some but not all of the area to be printed. The orifices in the array are arranged in a pattern such that adjacent parallel lines on the print medium are served by orifices that have positions in the array along the direction of the print lines that are separated by a distance that is at least an order of magnitude greater than the distance between adjacent orifices in a direction perpendicular to the print line direction.
Implementations of the invention may include one or more of the following features. Each of the orifice plates may be associated with a print head module that prints a swath along the substrate, the swath being narrower than the target width of the substrate. The number of orifices in each of the orifice plates may be within a range of 250 to 4000, preferably between 1000 and 2000, most preferably about 1500. There may be no more than five swath arrays, e.g., three, to cover the entire target width.
Other advantages and features will become apparent from the following description and from the claims.